Porous media are commonly used to filter, absorb, adsorb, separate, or otherwise remove selected components from a liquid. In the medical field, porous media are commonly used in the processing of biological fluids. For example, porous filter membranes have been used to separate components of biological fluid. One example of such an application is the Autopheresis-C® System sold by Baxter Healthcare Corporation, of Deerfield, Ill., the assignee of the present application. In the Autopheresis-C®, plasma is separated from the remaining blood components of a donor by passage through a spinning filter membrane. The plasma is collected for later processing and the other blood components are returned to the donor.
To be effective in the filtration, sorption, separation or removal of selected components from liquid, the porous medium must be “wettable” by the liquid with which the medium is in contact. “Wettability” is commonly determined by whether or not the liquid is absorbed by the porous medium. For example, a porous medium that, when contacted by a liquid, results in beads of liquid forming on the surface and remaining in bead shape is considered “not wettable” by the liquid.
Tests have been developed which characterize porous media in terms of whether or not they will be wet by a particular liquid. One way in which such media can be characterized is by their so-called “critical wetting surface tension,” or CWST, described in detail in U.S. Pat. Nos. 4,880,548 and 4,976,861, assigned to the Pall Corporation and incorporated by reference herein. The CWST of a porous medium may be determined by individually applying to its surface, preferably dropwise, a series of liquids with surface tensions varying by 2 to 4 dynes/cm, and observing the absorption or non-absorption of each liquid. The CWST of a porous medium, in units of dynes/cm, is defined as the mean value of the surface tension of the liquid which is absorbed and that of a liquid of neighboring surface tension which is not absorbed.
It has generally been assumed that porous media with comparable CWST values will exhibit similar performance characteristics, such as the rate of liquid flow through the medium at a given trans-medium pressure. However, it has been observed that porous media made of the same materials with the same mean pore size, same thickness and same CWST do not always behave in the same way. For example, porous media with very similar, or even the identical characteristics described above, may display significant differences in the interaction of the medium and the liquid with which the medium is in contact. For example, as discussed in greater detail below, in the field of biological fluid processing, similar porous media may at times display differences in the flow and the flow rate of the biological fluid through the media.
Until now, these differences in characteristics (such as, but not limited to flow rate) have not been discernible by any known method of characterizing the porous medium, such as CWST or other known tests. Thus, it would be desirable to provide a method of characterizing porous media with more precision and thereby discern differences between the surface characteristics of comparable media. Furthermore, a method that can discern subtle differences in surface characteristics can be correlated to certain desirable or undesirable interactions with the subject liquid, allowing the end user to select one porous medium over another otherwise comparable medium.